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Main Sequence Stars


Generally stars spend most of their active lives traveling through the stage of stellar life-cycle known as the Main Sequence. I say 'active' life because the remnants of dead stars can persist more or less indefinitely, slowly fading into obscurity. I say 'generally' because even dead stars sometimes find ways to obtain new life (see stellar accretion for example).

Main sequence stars fall into several spectral classes, as shown above. Going from top left to bottom right we have, a small dim red dwarf (spectral class M), an orange dwarf (spectral class K) a yellow dwarf, like the Sun (spectral class G), white stars (spectral class F on the left and the larger class A on the right) and the large hot and blue B class stars, bottom left, and O class stars, bottom right.

    A useful pneumonic: Oh Be A Fine Girl/Guy And Kiss Me

In addition, within each spectral class, there are ten spectral subclasses, ranging from 0 to 9, with subclass 0 hotter than subclass 9. Thus we have classes like G2, the Sun (Sol) is a G2 star. The lower subclasses, toward 0, are also called early spectral types, and those toward 9, later spectral types.

The colors of these stars depend upon the surface temperature, with red being the coolest, followed by orange, then yellow, then white and finally blue. In fact the blue B and especially the O class stars are so hot that much of their energy is emitted as ultraviolet radiation that is invisible to human eyes.

The temperature, and hence color of a star, is dependent largely on the star's mass. The table below illustrates the masses, radii and luminosities of each main sequence star class; mass, radius and luminosity are given relative to that of the Sun (1), so a B class star is some 500 000 times more luminous than the Sun, temperature is given in degrees K (to convert to degrees C subtract 273, which makes a negligible difference here), MS lifespan is the time spent on the main sequence:

Spectral Class
MS Lifespan yrs)
M red 0.1 0.1 0.001 3 000 100 billion
K orange 0.5 0.3 0.03 4 500 15 billion
G yellow 1 1 1 5 500 10 billion
F white 1.5 1.2 5.0 7 000 5 billion
A white 2.5 2 50 9 000 400 million
B blue 10 5 10 000 17 000 (10 to 100) million
O blue 40+ 20 500 000 40 000 (2 to 8) million

Note that the color is a 'spectral color', meaning that an orange star outputs more orange light than other spectral colors and so can appear orange. However, color is a perception, not a physical property of light (although it correlates to frequency) and when viewing a star such as the Sun in the vacuum of space it may appear white to the human eye. The Sun is a hot and bright G-class star (G2) and hence very pale yellow, but its brightness may white out the human vision, though its light is tinged pale yellow. When seen through the Earth's atmosphere, the Sun's light is both dimmed and reddened and so the Sun appears a definite yellow. The brightest star in Libra, Beta Librae or Zubeneschamali is rare in being tinged green, for unknown reasons.

So, more massive stars are larger, hotter and much more luminous. These figures are ball-park figures, since each class overlaps with the adjacent classes on either side, such that there is a whole continuous spectrum of star masses from about 0.08 to 150 times the Sun's mass. In fact the upper limit is not well established, but once stars reach 30+ solar masses they become unstable and tend to rapidly lose their excess mass by blasting it off into space. Stars smaller than about 0.08 solar masses are not hot enough to sustain nuclear fusion and so shine very briefly, if at all.

Also dependent upon the mass of the star is the stars longevity (that is the length of time that it spends on the Main Sequence). This can be estimated from the mass (M) and luminosity (L) using the approximation:

  Main sequence lifetime = 10-10 x (M / L) yrs,

  (with M and L in units of solar mass and solar luminosity, both equal to 1 for the Sun)

A spectral class G star, which is a yellow dwarf and is the class to which the Sun belongs, burns for about 10 billion years (10 000 million years) before it runs out of fuel. The Sun is currently middle-aged and should burn for another 5 billion years or so. Smaller stars, like the class M red dwarfs, may have less fuel to start with, but they burn so dimly that their fuel lasts much longer and they have lifespans that exceed the current age of the Universe. (The age of the Universe is established at 12.5 billion years and red dwarfs may burn for about 6 thousand billion years). At the other extreme, giant blue O and B class stars only burn for a few million or tens of million years. Thus, wherever O and B class stars are found, new star formation has recently occurred. Such regions include the bright blue spiral arms of the Milky Way Galaxy.

Most of the stars that you can see with the naked eye are the luminous white and blue stars. It is difficult to ascertain the numbers of red dwarfs since they are so dim that they are hard to detect from great distances.

O stars and B stars, the UV-stars

These are the very hot O and B giant stars (blue giants) whose peek output is not in the visible spectrum at all, but in the ultraviolet (UV). The heaviest O stars are blue hypergiants. O stars are very hot and massive blue giants that are so hot that helium becomes ionized in their outer atmosphere (helium, He, is a noble gas and very stable with a high ionization energy, meaning that it is very hard to remove electrons from helium atoms to form positive helium ions) and they have strong spectral lines of singly ionized helium, He II or He+. These stars are very rare, and as of the year 2000, no O0, O1 or O2 and only a few O3 and O4 stars had been discovered, with most known O stars being in the cooler O5 to O9 subclasses. However, more massive stars are being discovered all the time. These stars have short lifespans of 3 to 5 million years or so, increasing if the stars lose enough mass in their very intense stellar winds. O and B stars occur in the spiral arms of galaxies, giving them their bright blue color. In these regions, recent star formation has occurred, as these stars are absent from older regions, having already left the main sequence during stellar death. O and B stars are often found in small groups, called OB associations. Associations are loose groups of new stars, ranging from a few to several hundred parsecs across, often found outside central open clusters.

B stars have spectra dominated by the absorption lines of neutral helium (He
I) Be stars (e for emission) are surrounded by shells of gas that, being heated by the central star, emit radiation as the atoms de-excite.

A Stars

These are massive and very luminous white giant stars and are also quite short-lived. Being young they are often rotating rapidly and so appear stretched along their equator, like rugby balls (ellipsoid). Many A stars are rotating close to the maximum rate, beyond which they would fly apart! A model of such a star is shown below:

Rapidly rotating A Star

Examples of A stars are Vega, Sirius, Deneb and Altair. The spectra of A stars are dominated by hydrogen Balmer absorption lines (see atomic spectra) which are strongest in A0 to A3 stars, with lines of ionized calcium dominating in later spectral types. The effective surface temperature of these stars ranges from about 7500 to 9900 K.

AM Stars

these are stars of spectral type A0 to F0 that are slow rotators (and so more spherical) and have weaker calcium lines, but stronger iron lines and very strong rare earths and heavier elements - a heavy metal anomaly. Most are short-period binary stars that co-orbit another star. The tidal forces between the two stars act like a break, and this tidal breaking has slowed the rotation of these stars and increased the stability of their atmospheres, such that heavier elements, like iron and the rare earths, have diffused up from the core where they are generated by nuclear reactions. These high atmospheric concentrations of heavier elements account for the anomolously strong spectral lines of these elements (spectral absorption lines are generated in the star's upper atmosphere).

AP Stars

these are stars of spectral types B5 to F5 and have strong magnetic fields. They are also slow rotators, but they are single rather than binary stars. They have increased concentrations of Mn (manganese), Si (silicon), Eu (Europium), Cr (chromium) and Sr (strontium) in their atmospheres and with surface temperatures decreasing in the same order from Mn-stars to Sr-stars.

F Stars

These stars are white to white-yellow with strong spectral absorption lines due to singly ionized calcium (Ca II).
Examples include: Canopus, Polaris, Procyon. The effective surface temperature of these stars ranges from 6000 to 7400 K.

G Stars

These are yellow stars (yellow dwarfs), with an effective surface temperature ranging from 4900 to 6000 K. Spectral absorption lines of singly ionized calcium are dominant. (Ca+ or Ca II has two prominent spectral lines, one, the H line, occurs at 393.4 nm and the other, the K line, at 396.8 nm (nm = nanometer = 1 x 10-9 m or one billionth of a meter). Both these lines occur on the edge of the visible (violet) / ultraviolet part of the spectrum). Most other metals are non-ionized at these temperatures and give rise to neutral metal absorption lines. Some simple molecules are stable enough to form in the atmosphere at these temperatures, and molecular bands (molecules have many spectral lines that group into closely-spaced bands) of CH (carbon monohydride) and CN (carbon mononitride). Examples include  Sol (the Sun) and Capella.

K Stars

These are orange dwarfs with effective surface temperatures in the range 3500 to 4900 K. They have strong absorption lines of both Ca II and neutral non-ionized calcium, Ca I. (At these cooler temperatures, not all the calcium is ionized) and absorption lines due to other neutral metals are dominant and molecular bands are stronger and increase in strength from hotter K0 stars to cooler K9 stars. Examples include: Arcturus and Aldebaran.

M Stars

these are (relatively) cool red dwarfs. (Red giants and supergiants typically have spectral type M or S, but these are not main sequence stars). The effective surface temperature ranges from 2400 to 3480 K. Oxygen is in excess in their atmospheres, in the form of molecular oxides, such as Ti (titanium(II) oxide) and VO (vanadium(II) oxide) that give rise to molecular absorption bands. Neutral metal absorption lines are also strong, since temperatures are not high enough to ionize most metals.

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Article updated: 18/12/2023